It has been known for many years that silver is a useful antibacterial agent with broad-spectrum activity together with compatibility with mammalian tissue, and there have been many proposals to incorporate silver into wound dressings to obtain the advantage of the bactericidal properties of silver in a wound dressing. In addition, silver has been applied to fibrous material previously for non-wound dressing purposes, usually for the purpose of enhancing electrical conductivity, see for example UK-A-927,115, WO-A-92/16589, DE-C-2,639,287, U.S. Pat. Nos. 5,302,415, 5,186,984, 4,752,536, 4,643,918, JP-010207473A, and JP-020153076. Silver has been applied to such fibres, which are generally not gel-forming, in a variety of ways in those references, some of which involve immersing the fibres into a silver solution, but detail of the procedures used is often lacking.
Carboxymethyl cellulose, in particular carboxymethylated lyocell, has the ability to absorb a great deal of water and to form a gel on its surface. This property of the material has been found to be particularly advantageous in the formation of wound dressings that are both absorbent and gel-forming. The carboxymethylation of cellulose is described in WO-A-93/12275, and the use of carboxymethyl cellulose for wound dressings is described in WO-A-94/16746. Calcium (or sodium/calcium) alginate is another material useful in the formation of wound dressings, because of its absorbency and gelling capability. Gel-forming fibres for use in wound dressings are water-absorbent fibres which become moist and slippery or gelatinous upon the uptake of wound exudate and thus reduce the tendency for the surrounding fibres to adhere to the wound. The gel-forming fibres may also swell. Gel-forming fibres can be of the type which retain their structural integrity on absorption of exudate or can be of the type which lose their fibrous form and become a structureless gel or solution on absorption of exudate. GB-A-1,328,088, WO-A-91/11206, WO-A-92/22285, JP-A-4146218 and WO-A-02/36866 disclose the incorporation of silver into calcium sodium alginate, WO-A-01/24839 discloses carboxymethyl cellulose fibres containing silver chloride and WO-A-02/43743 discloses incorporating silver into a polymer which can be carboxymethyl cellulose or an alginate, the contents of these documents being incorporated by reference herein.
There have, however, been particular problems associated with the use of silver in wound dressings because of the fact that silver compounds are light-sensitive and darken on exposure to light. This can result in the production of products which have an unattractive visual aspect, even if they are technically suitable for use as wound dressings.
There are three particular aspects of the darkening of the silver compound in light which need to be addressed when seeking to produce a commercially acceptable silvered wound dressing. One aspect is the actual colour of the product, namely the need to have a product having a colour acceptable to the consumer. The second aspect is the need to produce a product having a uniform appearance. The third aspect is the stability (shelf-life) of the colour of the dressing within whatever packaging is used to package the dressing. If fibres are blended, and both fibres are white, any imperfection in the blending is not noticed by the consumer. Whilst this is primarily a visual issue, extremes of streakiness or discoloration within a wound dressing could be an indicator of incomplete or inadequate silver additions to the wound dressing or parts of it, or even the presence of excessive amounts of silver in some areas, which could indicate potential problems in use. Silver as an antibacterial material should be used in measured dosages, and this would not be the case if the silver level varied from dressing to dressing.
Kiers are well known for use in fibre treatment processes; for example, it is well known that cellulosic fibre can be dyed by being placed into a kier and the dye liquor pumped through the kier to give a product having a uniformly dyed appearance. A kier is a sealed container having inlets and outlets; it is capable of being pressurised and heated if required and is incorporated into a circuit such that liquor can be pumped through the kier. Located within the kier is a porous basket in the form of a stainless steel mesh and the product to be treated is packed consistently in the mesh to ensure uniform and even flow of liquor through the mesh during the pumping process.
Attempts were made, therefore, by the applicant to apply the silver to carboxymethyl cellulose fibre in a kier just as though the carboxymethyl cellulose was being dyed with a dye, using a solution of a silver-containing compound (industrial methylated spirit (IMS), H2O+AgNO3) in the kier. The silver-containing liquor was pumped from the centre towards the periphery. In that procedure a basket containing 1.25 kg of the carboxymethyl cellulose fibre was placed within the kier prior to charging the kier with liquor. Then 10.4 litres of a solution of (IMS, H2O+AgNO3) made up of 6.41 IMS, 4.01 water, 25 g AgNO3 at a silver concentration of 0.240 w/v and a temperature of 30° C. were pumped through the kier, which had a capacity of about 12 litres. After the liquor had been pumped around the circuit for 30 minutes, the liquor was drained out of the fibre and the product was moved immediately to subsequent stages, including the application of a textile finish and drying. After all of the subsequent treatments had taken place the fibre was removed and exposed to light. It was found that the fibre was not uniform in its silver take-up. The product produced by this process was found to be very streaky.
Attempts were therefore made to alter the kier process by using an upward-flow basket which fed the liquor from below rather than from the centre. Such a procedure was found to improve the product in the sense that it was less streaky but there was a distinct gradation of silver take-up from the bottom to the top of the basket.
It is an object of the present invention to provide an anti-bacterial wound dressing based on or derived from silvered gel-forming fibres in which the above disadvantages are reduced or substantially obviated. It is a further object of the present invention to provide silvered fibres for an antibacterial wound dressing based on silvered gel-forming fibres in which the above disadvantages are reduced or substantially obviated.